Heat 2 tablespoons of the oil in a pan. Add the onion and pepper and sauté for 5 minutes, then add the garlic and chilli and cook for a further 2 minutes without burning. Reduce the heat, pour in the vinegar and stock and cook gently for 10 minutes.
Season the flour on a plate. Dip the fillets in and dust all over. Heat the remaining olive oil in a large pan and cook the fillets for 3 minutes on each side. Serve at once with the spicy sauce.